Compositions having the general formula BaTiO3- ？wt% Ag, where , and 2 have been prepared by solid state ceramic processing and sintered at 500 and for 5？h. Thermogravimetric analysis (TGA), X-ray powder diffraction (XRD), infrared absorption spectra (IR), and scanning electron microscopy (SEM) were used to characterize the obtained sensor pellets. It was found that no solid state reaction took place between BaTiO3 and CuO during sintering process. The sensitivity of the prepared sensors to CO2 gas increases with increasing sintering temperature and Ag content. The correlation between Ag content at different sintering temperature and structure characterization is discussed. 1. Introduction Atmospheric pollution is defined as a status containing gases, offensive odors, and particles that are harmful to human, animals, vegetables, or living environments above the regulation limits in specific regions [1]. A severe phenomenon of the environment is global warming. This phenomenon is considered to be related to the aspect that the concentration of carbon dioxide in the atmosphere is increasing. The ideal chemical gas sensor must be chemically selective, reversible, fast response, highly sensitive, no contaminating, no poisoning, simple using, small size, simple fabrication, relative temperature insensitivity, low noise and low manufacturing costs [2]. While in all the mentioned cases it would be valuable to have a low-cost sensor. Nonexpensive and robust detection systems are required for air quality, food control, and early fire detection. Gas sensors monitoring CO2 concentrations are used in fields such as agricultural industries, biotechnological processes, air conditioning, medical services, housing, and environmental observation [3]. Most of the currently available sensing systems are based on optical detection, which makes them expensive. Some cheaper electrochemical sensors have also been developed, but their fabrication process is still complicated. Moreover, in the presence of humidity, these sensors reduce drastically their response to CO2. In this situation, solid-state gas sensors based on semiconductor metal oxides may be a promising alternative, since they offer good sensor properties and can be easily mass-produced. Semiconductor gas sensors based on the capacitance or resistance change of the sensor are good candidates to reach a reliable and cheap sensor for CO2. The gases’ adsorption or their reaction on the surfaces of the semiconducting materials induces the change in the density of the conducting electrons in the polycrystalline sensor
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